The present invention relates to an antenna feed network, and to an antenna such as a quadrifilar helix antenna incorporating such a feed network.
A conventional feed network is shown in U.S. Pat. No. 5,541,617. A hybrid junction power divider feed circuit provides 0 to 180xc2x0 phase shift. The radiating elements are connected to the feed circuit in pairs. The second element of each pair is shorter than the first element by a predetermined distance to provide a phase quadrature between them.
An alternative feed network is shown in FIG. 5 of U.S. Pat. No. 5,955,997. The feed network is a non-isolating inline power splitter with an excess quarter-wavelength line in one output arm to generate the required 90xc2x0 phase differentials.
A helical antenna is described in U.S. Pat. No. 6,172,656. In the embodiment of FIG. 3, the antenna arms are aperture fed by a drive circuit including a 180xc2x0 hybrid coupler and two 90xc2x0 hybrid couplers.
An exemplary embodiment of the invention provides an antenna feed network including:
a 180xc2x0 hybrid coupler having a feed port, a 0xc2x0 port, and a 180xc2x0 port having an approximately 180xc2x0 phase difference with the 0xc2x0 port;
a first antenna port coupled to the 0xc2x0 port;
a second antenna port coupled to the 0xc2x0 port via a respective phased line, the second antenna port having an approximately 90xc2x0 phase difference with the first antenna port;
a third antenna port coupled to the 180xc2x0 port; and
a fourth antenna port coupled to the 180xc2x0 port via a respective phased line, the fourth antenna port having an approximately 90xc2x0 phase difference with the third antenna port.
It will be appreciated by a skilled person that the actual phase difference between the ports may differ slightly from the approximate values given above: in practice the phase difference may vary by up to 2%, or even in extreme cases up to 5% either side of the approximate value.
The feed network may be incorporated into an antenna in which a radiating element is coupled to each antenna port. A preferred application for the antenna is for receiving satellite Global Positioning System (GPS) signals.